It may have taken six seasons’ worth of cruelties from Lady Mary, but in the penultimate episode of Downton Abbey, which aired Sunday in the U.S., Lady Edith finally took off her white tea gloves and let her awful sister have it. The tell-off was so satisfyingly nasty that it could have earned Edith her own Real Housewives of Yorkshire spin-off had Mary not disqualified her from eligibility by telling Bertie about Edith’s secret daughter, detonating Edith’s latest chance at marriage. For those who would like a refresher in Edith’s consequent kiss-off, take a look at the scene below.

But why would Mary be so especially evil in the first place? In an interview with PBS, Michelle Dockery, who plays Mary, attempted to rationalize her character’s heinous actions.

“Well, in the previous episode, the whole [tragic car crash] has happened with Charlie and it brings back all those memories [of Matthew’s death] and it’s almost like a kind of second grief that happened. . . . She can’t handle it. And so what happens is she just takes it out on her sister which is often what she does. . . . And then of course [the proposal] happens and [Edith] becomes potentially this great lady and out ranks us all. And Mary can’t bear that.”

Although the scene was excruciating for both characters, Dockery revealed that she and Laura Carmichael, who plays Edith, were thrilled to sink their teeth into the loaded exchange.

“When we read it, me and Laura, I remember texting her going, ‘Oh, my God. We’re gonna have such a good time.’ Cause we love it when [creator] Julian [Fellowes] writes these great scenes between the sisters. It’s one of the core relationships in the show.”

Although Dockery will defend Mary until the end, the actress understands that viewers were likely pleased to see Edith call Mary a bitch—twice. “I’m sure audiences out there applauded that [Edith] said it cause [Mary] had it coming,” Dockery allowed.

Edith’s profanity-laced meltdown, while much deserved, does defy social etiquette of the time, and David Evans, who directed the episode, discussed the “very strange atmosphere” while filming that exchange.

“Particularly for the women who are in the cast of Downton Abbey—they’re always acutely conscious that they’re playing women from another age,” explained Evans. “The question is constantly, ‘Can I go there? Can I be that off the leash?’ Obviously, the way that Downton Abbey is written, there are very few scenes where people are completely out of control and have lost their temper and are saying things really unguardedly like that. . . . Both of them were feeling, ‘Is this too big? Is this too much?’ And I was basically going, ‘Nothing is going to be too much in a scene like this.’”

The blow-up does have a resolution ultimately, though, with Edith taking the high road and returning to Downton for Mary’s wedding. Explaining why she returned, Edith says, “Because in the end, you’re my sister. And one day, only we will remember Sybil. Or Mama, or Papa, or Matthew, or Michael, or Granny, or Carson, or any of the others who have peopled our youth. Until at last, our shared memories will mean more than our mutual dislike.”

Those three sentences were so poignant, Dockery revealed, that they actually brought her to tears in real life.

“There were such strong parallels in that dialogue,” Dockery said. “Laura and I have this closeness, and memories that we will only share. When she said those words, there was a double meaning to it. I was really tearful. . . . You spend so much time with one another. You become so close. And we knew that these would be our last scenes together and it was just really emotional. It was amazing actually.”